This only just came to me over morning coffee. I don't have time to dwell on it this morning, and it is so nascent I may forget it if I don't record it. So, here goes . . .
Start with the four main classes, cleric, fighter, magic-user, thief. Everyone selects one at character generation like always. But instead of limitless levels, or a cap at 10, 14, or whatever, each basic class only goes to 3rd level. At 4th level you essentially choose a new class.
Here's the thing, though: the new class is a natural progression of the old class. So, someone who starts as a fighter would progress to a "fighting" class. Say, a ranger, or a paladin, or barbarian. There could be a lot of these classes. Progress through three levels of this new class, then change again. Subsequent changes become increasingly restrictive, based on the "class path" up to that point. That isn't to say you couldn't go back to a "branch point" and start along a different path, there would just be some sort of penalties for doing so.
I'm not sure if this would work with OD&D. I like the idea of it, though. Hopefully I'll have time to develop it, and see how it shapes up.